The present invention relates to a drive arrangement for a yarn storage and dispensing unit serving as a yarn buffer and delivery unit for textile machines, said arrangement comprising a rotatingly driven cylindrical winding member on which the yarn is adapted to be wound up in juxtapositioned windings and from which the yarn leaves at the axial side far from the yarn supply point to be fed to the textile machine, wherein the yarn coil is adapted to be axially shifted against the action of a frictional force so that the drive motor is braked by this frictional force in correspondence with a yarn coil of desired length.
In practice, there are frequently substantial variations of the length of yarn to be withdrawn from the yarn coil via the yarn storage and delivery unit within the unit of time; accordingly the yarn often is not withdrawn in a proper manner. According to a prior proposal the yarn windings have been pushed on the winding member by a toothed disc driven by the winding member and having an axis of rotation inclined relative to the axis of rotation of the winding member. The winding member itself is driven by a motor. This arrangement makes use of the fact that a counter force is developed between the yarn windings and the toothed disc which force, as defined above, provides for a braking torque acting on the winding member and increasing with the length of the coil, and that the amount of yarn stored on the winding member merely can increase until the braking torque reaches the driving torque of the motor.
When a driving motor is used the torque of which in the usual manner decreases with increasing motor speed the amount of yarn stored on the winding member of the yarn storage and dispensing unit will become smaller at high yarn withdrawal speeds whereas this amount increases at low yarn withdrawal speeds. Accordingly there is the risk that at very high yarn withdrawal speeds the yarn storage and dispensing unit can no longer fulfill its intented buffer function whereas at low withdrawal speeds excessive yarn is wound up and two or more yarn layers are superimposed thereby preventing a uniform, smooth yarn supply to the textile machine.
Therefore it has been proposed to drive the winding member of such a yarn storage and dispensing unit by a motor providing for a torque which is substantially constant within the range of the operational speeds. However, it turned out to be difficult to obtain the desired constancy of the motor torque.